RTT Client hint request header field provides the approximate round trip time on the application layer, in milliseconds. The RTT hint, unlike transport layer RTT, includes server processing time.
|Header type||Request header, Client hint|
|Forbidden header name||no|
The RTT value is rounded to the nearest 25 milliseconds to prevent fingerprinting; There are many other mechanisms an attacker might use to obtain similar round-trip information.
The hint allows a server to choose what information is sent based on the network responsiveness/latency. For example, it might choose to send fewer resources.
Vary header is used in responses to indicate that a different resource is sent for every different value of the header (see HTTP Caching > Varying responses). Even if
RTT is used to configure what resources are sent consider omitting it in the
Vary header — it is likely to change often, which effectively makes the resource uncachable.
The approximate round trip time in milliseconds, rounded to the nearest 25 milliseconds.
A server first needs to opt in to receive the
RTT header by sending the
Accept-CH response header containing
Then on subsequent requests the client might send an
RTT header back:
|Network Information API |
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